Compact fluorescent lamps (CFLs) are fluorescent lamps that have been folded several times, allowing them to fit into a smaller space. This allows them access to the market which was previously the exclusive domain of incandescent lamps (ILs). The CFLs are desirable replacements for ILs primarily because they are more efficient light sources. Today, the range of power and light output of CFLs is 5 to 55 W with 250 to 4800 lumens.
CFLS are comprised of a glass envelope with a phosphor coating on its interior surface. Each end of the lamp has an oxide coated electrode, the oxide coating serving to enhance electron emission. The glass envelope, during operation, has about a six millitort vapor of mercury (Hg) and several Torr of a rare gas, e.g., argon (Ar). A low pressure discharge is maintained between the two electrodes, causing the Hg to emit ultraviolet (UV) radiation. The radiation is converted to visible light by the phosphor coating.
As indicated above, CPLs are much more efficient at converting electrical energy to visible light than ILs. Typically, a CPL delivers 50 to 60 lumens per watt (LPW), while the efficiency of an IL is 16 LPW. Thus, replacement of ILs with CPLs yields substantial energy savings. Furthermore, ILs have a lifetime of only about 750 hours, while CFLs last from 6,000 to 10,000 hours. In commercial venues, where replacement of light bulbs involves labor costs, the less often a bulb needs to be changed the more economical the installation. Finally, the electric utilities desire to reduce the peak load demand by supporting energy conservation. Building additional power generating facilities or running less efficient auxiliary generators to accommodate the peak load is costly. Therefore, utilities have found it more economical to support energy saving devices to reduce peak load. This is called Demand Side Management, or DSM. These DSM programs have given a large boost to the entire field of CFLs.
Reducing the size of CFLs to expand their applicability is a goal of the lighting industry, but the problems of thermal management become increasingly prohibitive as the dimensions of the lamp shrink. The performance of CFLs is strongly dependent upon the Hg pressure in the lamp, which increases with temperature. With a typical ambient temperature of 25.degree. C., some of the heat generated by the discharge beneficially warms the cold spot to the ideal temperature of 40.degree. C. At this temperature, the vapor pressure of Hg delivers the maximum UV radiation to the phosphor coated walls. Standard fluorescent lamps (FLs) have been engineered to operate at the ideal temperature. But the domain of CFLs is in compact applications. Therefore, to attain the desired luminous flux from a CFL while maintaining its compactness requires that the wall loading, or power per unit surface area, be increased over that from standard FLs. This causes the cold spot temperature of the CFLs to rise beyond the ideal of 40.degree. C., and the efficiency of the lamp drops.
This problem can be solved by either of two methods. A region of the glass envelope can be cooled by changing its geometry or by heat sinking it. Because the Hg vapor fills the entire volume of the glass envelope, cooling any small portion of that envelope will effectively control the Hg pressure anywhere in the lamp. This method has the disadvantage of constraining the possible geometries available to the lamp designer. Furthermore, application of the CFL in a fixture may obviate advantages gained by altering the geometry. The other solution is to use an amalgam of Hg and indium, which has a lower vapor pressure than Hg itself..sup.1 Without an amalgam, the efficiency of a CFL is within 10% of its optimum over a narrow 25.degree. C. range centered at 40.degree. C. With an amalgam, that range is shifted to higher temperatures, specifically tailored to those encountered in a CFL, and the efficiency is within 10% of its optimum over a range of 40.degree. C.-120 C. This makes the lamp both efficient at the nominal operating temperature of the lamp and makes it insensitive to departures from the specified operating temperature. Unfortunately, the Hg pressure takes longer to become established because the lamp has a longer warm-up time. This delays the time at which the lamp attains its maximum light output. FNT .sup.1 J. Hoffman, "Compact, Single Ended Fluorescent Lamp with Fill Vapor Pressure Control", U.S. Pat. No. 4,694,215.
An additional malady resulting from overheating is the degradation of the electronic ballast. The addition of an integral electronic ballast to CFLs expands their applicability, but it also thermally couples the lamp to the electronics. This, and the compactness of the source, causes the temperature of the components to rise and shortens their useful life.
As the size of the CFL is reduced, the phosphor loading (power per unit surface area covered with phosphor) increases, leading to faster phosphor light output deterioration. This is due to the density of damaging species that impinge on the phosphor. Hg+ ions tend to sputter the phosphor and implant themselves, causing darkening, which inhibits the generation and transmittance of visible radiation. High energy radiation can also damage the phosphor. In particular, the 185.0 nm Hg radiation is somewhat damaging. Under normal operating conditions of the CFL lamp, i.e., cold spot temperature of 40.degree. C., the amount of 185.0 nm radiation is only about 6%. However, as the temperature goes up this percentage increases to as much as 20-30% depending on the temperature. Phosphors have been improved so that they can withstand a higher wall loading, yet they remain a weak link in the longevity of CFLs. One method of protecting the phosphor is to coat it with a thin film of alumina..sup.2,3 This coating is transparent to UV, allowing the UV to strike the phosphor and generate visible light while shielding the phosphor from damaging species. FNT .sup.2 A. G. Sigai, K. A. Klinedinst "Phosphors with Improved Lumen Output and Lamps Made Therefrom", U.S. Pat. No. 5,309,069. .sup.3 F. R. Taubner, A. G. Sigai, C. Chenot and H. B. Minnier "Method for Preparing Zinc Ortho Silicate Phosphor Particle", U.S. Pat. No. 5,196,234.
Packaging remains a problem for CFLs. They are not a suitable retrofit in many incandescent applications. They do not fit into A19 or A23 dimensions. This is especially true for 100 W incandescent equivalent CFL's. As it turns out, this is a very popular table and reading lamp because of its high lumen value (.about.1700 lumens). Therefore, a compact, small size 1700 lumen CFL would find many industrial, commercial, and residential applications. Generally, CFLs are shaped as a cluster of 2, 4, 6, or even 8 parallel tubes interconnected to allow one continuous discharge. Or at times they are a series of U shaped tubes. They are perched atop an integral electronic ballast, adding further to their overall length. An incandescent lamp with an E27 cap has an overall length of about 108 mm, whereas a 23 watt/1550 lumen triple-U lamp still has an overall length of 173 mm. This seriously hinders the use of CFLs as retrofit replacements for ILs.